The present invention relates to a flow meter for measuring the flow of a fluid, such as air or liquid running in a measuring tube, accurately throughout a wide range.
Among flow meters for measuring the flow of a fluid which runs in a measuring tube, a Karman-vortex flow meter is known. A conventional Karman-vortex flow meter disclosed in Japanese Patent Laid-open No. 60-40914 develops a Karman vortex in the flow of a fluid, and a generating frequency (referred to as a frequency hereinafter) of the vortex is measured for calculating the rate of the flow. The calculation is based on the fact that the Karman-vortex generating frequency is proportional to the flow. For measuring the Karman-vortex frequency, the meter disclosed in the Japanese Patent Laid-open No. 60-40914 employs an ultrasonic or oscillation technique. It is known that when ultrasonic or oscillating waves directed to a Karman vortex have a frequency or phase change, the change in the ultrasonic or oscillating waves may be measured with only a large, complex, expensive meter even if the measuring tube is relatively small. Because the measuring accuracy of such an expensive meter depends primarily on the generating mechanism of the Karman vortex, the accuracy is easily reduced by a condition such as an ambient temperature or disturbing turbulence which affects the generation of a Karman vortex.
Japanese Patent No. 3113946 discloses that a Karman-vortex frequency is measured with a magnetic field. The modified flow meter will be explained. FIG. 7 is a cross sectional view of a conventional flow meter. The meter includes a measuring tube 1 in which an electrically conductive fluid flows and a vortex generator 2 provided in the measuring tube 1. The vortex generator 2 generates a Karman vortex 3. The meter also includes a pair of electromotive force measuring electrodes 4a and 4b, a detector circuit 5 electrically connected to the electromotive force measuring electrodes 4a and 4b for measuring a voltage between the electrodes 4a and 4b to calculate a flow rate of the fluid running in the measuring tube 1, and a pair of magnetic field generators 7a and 7b mounted around the measuring tube 1. The magnetic field generators 7a and 7b are two magnets mounted to both sides of the measuring tube 1, respectively, so that the two, N and S, poles are opposite to each other. More specifically, the magnetic field generators 7a and 7b are arranged so that the orientation of the magnetic field from the N pole to the S pole is perpendicular to the axis of the vortex generator 2 and to the electromotive force measuring electrodes 4a and 4b. Downstream of the vortex generator 7 in the flow direction, a pair of lines of Karman vortices are generated in which alternate vortices of opposite rotation are developed at a frequency proportional to the representative dimension of the vortex generator 2. The electromotive force measuring electrode 4b is located downstream of the vortex generator 2. The electromotive force measuring electrode 4a opposite to the electromotive force measuring electrode 4b is located downstream of the vortex generator 2 and upstream of the electromotive force measuring electrode 4b. FIG. 7 illustrates the electromotive force measuring electrode 4a arranged unitarily with the vortex generator 2 for simplicity.
The Karman vortex 3 generated by the vortex generator 2 changes the velocity of the flow thus causing a change in the magnetic flux of the magnetic field developed between the magnetic field generators 7a and 7b. The change in the magnetic flux then generates an inductive electromotive force between the electromotive force measuring electrodes 4a and 4b. The number of voltage changes is proportional to the number of vortices and is measured by the detector circuit 5 for calculating the flow.
However, as the electrodes in the conventional Karman-vortex flow meter are directly provided downstream of the vortex generator, they detect vortices in an area where the vortices do not depart completely from the vortex generator and before the vortices grow up to a measurable size. Therefore, the meter receives an influence of fluctuations of the Karman vortices.
Also, as the electrodes are arranged at a point and thus has a small sensing area, the accuracy of measurements may stay low. Particularly, when the electric conductivity is low, a small flow is hardly measured.
The meter is susceptible to disturbing noises and thus requires a scheme for diminishing the affect of noises. Having to include a sophisticated filter circuit makes the overall arrangement of the meter intricate and expensive, thus creating a secondary drawback.
Moreover, when the flow of the fluid is small where Reynolds number is less than 3000 calculated from Re=UL/v (where U is the average flow velocity in the cross section, L is the representative length, and v is the kinetic viscosity coefficient), the velocity distribution significantly varies by the resistance of the inner wall of the measuring tube. This makes Karman vortices generate unstable, thus reducing the accuracy and repeatability of measurements and reducing the accuracy of the calculation of the flow.
A vortex flow meter follows:
a measuring tube in which a fluid is carried;
a vortex generator provided in the measuring tube for developing a Karman vortex in the fluid;
a magnetic field generator for generating a magnetic field to be applied downstream of the vortex generator across the measuring tube;
a pair of electromotive force measuring electrodes provided downstream of the vortex generator for measuring an electromotive force which is generated when the Karman vortex passes across the magnetic field;
a pair of reference electrodes provided at locations upstream and downstream, respectively, of the electromotive force measuring electrodes for measuring potentials at the locations, respectively; and
a detector circuit electrically connected to the electromotive force measuring electrodes and reference electrodes for calculating the flow of the fluid from the electromotive force and the potential measured by the reference electrodes.
The flow meter measures the flow while offsetting a change of the flow caused by a change of the measuring environments and conditions, hence increasing the measurement range, decreasing the cost with no use of extra components for a noise reduction, and improving accuracy of measurement.